mogrify -auto-orient imagename.jpg
did the trick.
iOS recently added an option for web uploads form to use a smaller image size - which can save a lot of bandwidth, since often you don't need an image in its full multi-megapixel splendor. But apparently if you do that (as I tried with the first image I posted on my commonplace blog) the "-auto-orient" I was relying on gets confused. (I repeated the process with the full image it was fine.)
I haven't really researched this yet, but I suspect the Exif information - explaining how the image was shot rotated - is preserved for the smaller image, but the image data already has the appropriate rotation applied.
I know I'm not the only one wrestling with this kind of thing - here's an article on The dumb reason your fancy Computer Vision app isn’t working: Exif Orientation. As it says:
Exif metadata is not a native part of the Jpeg file format. It was an afterthought taken from the TIFF file format and tacked onto the Jpeg file format much later. This maintained backwards compatibility with old image viewers, but it meant that some programs never bothered to parse Exif data.But worse than that, right now I'm not convinced there's a flag that says "this rotation data is for historical purposes only" vs "this rotation data is needed for proper display of the image" - or if there is, iOS and (my admittedly old install of) ImageMagick don't agree what that is.
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